Measurement of the number of ultra-fine particles is prerequisite to fundamental study of the particles for air pollution measurement, and has also been applied to investigation of the cause why the ultra-fine particles have been generated in semiconductor clean rooms and the like so that the clean rooms can be kept at a clean state by removing the ultra-fine particles from the clean rooms. As well known in the art, an optical instrument such as a laser is utilized to measure the number of the ultra-fine particles. In general, a particle measurement limit of the optical instrument corresponds to about 0.1 μm in particle size. Thus, a condensation particle counter has been utilized for measuring the ultra-fine particles having their sizes of 0.1 μm or less beyond such a measurement limit. The principle of the condensation particle counter is that a liquid is condensed around the ultra-fine particles by utilizing the ultra-fine particles as condensation nuclei, and then, the ultra-fine particles are caused to grow to such a degree that they can be measured through the optical instrument.
In order to cause the liquid to be condensed around the ultra-fine particles, the following three types of technologies are currently utilized. The first one is an oldest technology. According to this technology, particles to be measured are injected into a container with water contained therein, the container is hermetically closed, and then, inside pressure of the container is rapidly reduced. Thus, inner temperature of the container is rapidly decreased, and temperature of water vapor within the container is consequently lowered. As a result, the vapor becomes supersaturated. The vapor starts to be condensed under such a supersaturated state, and is then condensed around the particles serving as condensation nuclei. After the condensation of the vapor has been completed, the particles become water droplets that the particles are enclosed in the condensed water. The particles can be easily measured through the simple optical instrument, because these water droplets are very large. However, since the measurement of the particles according to a conventional condensation particle counter in which the particles grow by such an expansion process should be intermittently made, there is a problem in that continuous measurement of the particles is greatly restricted. Accordingly, this technology has been hardly employed at present.
According to the second technology, hot air with saturated water vapor and cold air with the particles are mixed with each other. Thus, supersaturated vapor is formed in a region where the hot and cold air is mixed. Even in such a case, the supersaturated vapor is also condensed around the particles serving as condensation nuclei in the same way as the first technology. Such a type of condensation particle counter is called a mixing type condensation particle counter. However, very high supersaturation may be formed partly in the mixing type condensation particle counter, and thus, the vapor is spontaneously condensed into the water droplets even though the particles used for the condensation nuclei are not provided therein. Therefore, there is also a problem in that the measurement of the number of the particles is inaccurate. Accordingly, this technology has been utilized only in some restricted fields.
The third technology is a conductive cooling type condensation particle counter of which constitution is shown in FIG. 1. The constitution of the conductive cooling type condensation particle counter will be explained with reference to FIG. 1. Alcohol 12 is contained in a storage pool 10, and a cylindrical absorbing member 22 is attached to an inner wall of a saturator 20 which is integrally formed with and extended from the storage pool 10. The alcohol 12 is absorbed into the absorbing member 22 which is made of porous material such as nonwoven fabric and of which one end 22a is immersed into the alcohol within the storage pool 10, and thus, the other end 22b of the absorbing member is caused to be wetted by means of a capillary phenomenon. At an outer wall of the saturator 20 is installed a heater 26 for heating the alcohol permeated into the absorbing member 22 to about 35° C. A condenser 30 is located downstream of the saturator 20 and is provided with a thermo-electric cooler 32 which causes the condenser 30 to be kept at a temperature of about 10° C. for condensing alcohol vapor. In order to sense and measure the grown particles, a well-known optical instrument 50, which comprises an assembly of mirrors or lenses and utilizes a laser or a semiconductor laser as a light source, is located in the vicinity of a leading end of the condenser 30. Further, a flowmeter 60 for regulating a flow rate of the grown particles by means of opening/closing operation of a valve (not shown) and a vacuum pump 70 for sucking the grown particles thereinto are successively installed downstream of the condenser 30 in a state where a pipe 62 is interposed therebetween.
The operation of the conventional conductive cooling type condensation particle counter constructed as such will be explained as follows. First, air with the ultra-fine particles floating therein (hereinafter, referred to as “aerosol”) is supplied into the saturator 20, which is kept at the temperature of 35° C. by the heater 26, through an inlet 24 of the saturator 20, and then, it is saturated with the alcohol 12. The alcohol-saturated air continues to flow downstream, and it passes through the condenser 30 corresponding to a cold region of which temperature is maintained at 10° C. The alcohol-saturated air passing through the condenser 30 is supersaturated, and the alcohol is then condensed around the particles in the air so that the particles become grown. The grown particles become larger to their sizes of about 12 μm and are then discharged from the condenser 30. Thus, the optical instrument 50 can readily measure the number of the particles. Furthermore, the grown particles are sucked by the vacuum pump 70, and the flow rate of the particles sucked into the vacuum pump 70 is regulated by the flowmeter 60.
In case of the third type or conductive cooling type condensation particle counter, the thermo-electric cooler used for keeping the condenser at the low temperature of 10° C. is poor in view of a coefficient of performance for removing heat from the condenser by using electricity, and thus, the cooler is good for removing a small quantity of heat from the condenser but inappropriate for removing a large quantity of heat from the condenser. Accordingly, a capacity of the condensation particle counter used commonly and currently is about 0.3 to 1.0 liter/min since it is difficult to cool down a large quantity of air. In particular, in case of the semiconductor clean rooms where it is necessary to sample the particles at a high flow rate, a need for a condensation particle counter capable of sampling the particles at the high flow rate has been required for a long time. However, suitable equipment has not yet been developed. In addition, if the water is used as an operating liquid, the pure ultra-fine particles are merely discharged toward the outside of the condenser since the water vapor passing through the condenser is first condensed at an inner wall surface of the low-temperature condenser without a condensing process in which the vapor is condensed around the particles serving as the nuclei. Thus, there is a problem in that the ultra-fine particles cannot be measured through the optical instrument. That is, the conductive cooling type condensation particle counter has a disadvantage in that only the alcohol must be utilized as the operating liquid. In particular, since the alcohol becomes a pollution source, the conductive cooling type condensation particle counter in which the alcohol should be used as the operating liquid is not suitable for a semiconductor fabrication process.